This invention relates to a digital transmission system involving multilevel multiplexing of digital signals from different sources to form a single high-speed signal. Multilevel multiplexing involves the use of cascaded time division multiplexers in which the higher order multiplexers interleave the outputs of two or more lower level multiplexers to form a multiplexing hierarchy.
The prior art includes a digital transmission system known as the Synchronous Optical Network (SONET). This network comprises multiplexers that utilize interleaving techniques for combining low speed tributaries to produce a high-speed line signal. Further details of the SONET will be found in the following publications "Synchronous Optical Network (SONET)," Bell Communications Research Technical Advisory, TA-TSY-000253, April 1985; R. J. Boehm, Y. C. Ching, and R. C. Sherman, "Synchronous Optical Network (SONET)," Globecom 1985, pp. 1443-1450. Each tributary signal arrives at the multiplexer with a source or subscriber-imposed pattern of framing bits (or a byte) as well as overhead bits thereon. Since these systems are synchronous, each source or subscriber terminal has available the master system clock; thus tributary signals will be bit synchronous with the master system clock but will have random phasing of the aforementioned framing and overhead bytes, since these portions of the signal are subscriber-controlled.
Such randomly framed tributary signals can be combined in a single multiplexer without the addition of any multiplexer framing time slots or the use of any high-speed processing circuitry following the multiplexer. This design simplifies the circuitry and yields a high-speed line signal with a bit rate that is an exact multiple of the tributary bit rates and hence also of the master clock. Each multiplexer comprises a preprocessor in each tributary channel with the outputs of all preprocessors applied to a bit interleaver, the output of which is the high-speed signal. In the aforementioned SONET system the preprocessors simply locate the frame of each incoming tributary and rewrite some of the overhead bytes thereon so that each channel can be identified at the receiver. Each preprocessor does this independently of the frame alignment of its neighbors. Each tributary signal thus is unchanged in frame alignment by its preprocessor and hence the high-speed signal will comprise the multiplexed tributary signals with their original frame alignment which as stated is arbitrary or random. At the receiving ends of these prior art systems, the demultiplexer initially has a random phase with respect to the individual multiplexed channels. The demultiplexer frames on one of the channels and determines its identity by reading its overhead data. The phase of the demultiplexer can then be sequentially stepped until all channels are correctly aligned and demultiplexed.
Problems arise in extending this multiplexing concept to systems including higher orders of multiplexing. For example, each of the higher order tributaries arriving at a higher order multiplexer will comprise several randomly frame aligned subtributaries. Thus, in order to identify each subtributary and to read, modify or rewrite the overhead data therein, each higher order tributary must, in effect, be broken down into all of its subtributary components and then reassembled prior to application to the higher order multiplexer. Thus, each preprocessor of each higher order multiplexer would necessarily comprise the equivalent of a full blown demultiplexer and remultiplexer, in addition to its other circuitry.
Further, the extension of the multiplexing concepts to multilevel multiplexing systems results in an overly tight coupling between subscriber provided data and network operation that can cause problems. These problems include reframing activity throughout the system resulting from switching between subscriber terminals. Worse still, certain defective input signals can prevent proper starting of the network. Also, coordination and administration of scramblers needed to achieve good signal statistics becomes a problem with the simple cascading of prior art multiplexers of these types.